Histone proteins organize DNA into nucleosomes, which are regular repeating structures of chromatin. The acetylation status of histones alters chromatin structure, which, in turn, is involved in gene expression. Two classes of enzymes can affect the acetylation of histones —histone acetyltransferases (HATs) and histone deacetylases (HDACs). A number of HDAC inhibitors have been characterized. However, to date no effective candidate for cancer therapy has been identified. Therefore, there is a need in the art to discover HDAC inhibitors that have effective anti-tumor activity.